WASHINGTON (Reuters) - U.S. rights advocates said they are on the alert for ways in which votes could be suppressed in Tuesday's congressional elections because complaints about registration problems, faulty equipment and intimidation have been a feature of early balloting that began last month.

Democrats and advocacy groups say they have been grappling with a diverse crop of new voting restrictions in this midterm election cycle, which will determine whether or not Republicans keep control of the U.S. House of Representatives, the U.S. Senate and governorships of 36 states.

North Dakota introduced a voter ID requirement that Native Americans say discriminates against them, Kansas and Georgia moved polling locations and changes in Tennessee registration laws led to people being removed from the voting lists.

Advocacy groups said the changes stack the deck against minority voters likely to support Democratic candidates.

Each of those hotly-contested states' Secretary of State, the top election official, has said the changes were made to protect against voter fraud and accommodate budgetary constraints, not to suppress voting. Independent studies have found that voter fraud is extremely rare in the United States.

Tuesday's elections, widely seen as a referendum on Republican President Donald Trump, have been portrayed by both Republicans and Democrats as critical for the future of the country.

"We're seeing a tug of war for the soul of this country," said Jamal Watkins, who leads civic engagement at the National Association for the Advancement of Colored People, or NAACP.

"It's become the norm for a secretary of state who's conservative to use their position to suppress the vote, and that means we've hit a crisis point in our democracy," he said.

The intense political environment has led to a surge in interest from people offering to help monitor polling stations.

The NAACP and Common Cause belong to a coalition of more than 100 groups that set up a national hotline for voters to call or text to report irregularities or seek information.

Hotline traffic in recent weeks was higher than in the previous U.S. midterm elections in 2014, said Marcia Johnson-Blanco, an official with the Lawyers' Committee for Civil Rights Under Law, which is part of the coalition.

Common Cause said it has signed up more than 6,500 volunteers, compared to 3,000 in the 2016 presidential election.

"This work is needed now more than ever," Common Cause President Karen Hobart Flynn said.

(Reporting by Julia Harte in Washington; Editing by Jim Finkle and Grant McCool)

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